1. Field of the Invention
The present disclosure is directed to a method for preparing a partially demineralized bone graft. More specifically, this invention relates to a load-bearing osteogenic osteoimplant fabricated from a monolithic section of cortical bone and to a method for making the osteoimplant as well as a method of using same.
2. Description of the Related Art
Shaped or cut bone segments have been used extensively to solve various medical problems in human and animal orthopedic surgical practice and their application has also extended to the field of cosmetic and reconstructive surgery, dental reconstructive surgery, and other medical fields involving surgery of hard tissues. The use of autograft bone (where the patient provides the source), allograft bone (where another individual of the same species provides the source), xenograft bone (where another individual of a different species provides the source) or transgenic bone (where a transgenic species provides the source) is well known in both human and veterinary medicine. In particular, transplanted bone is known to provide support, promote healing, fill bony cavities, separate bony elements (such as vertebral bodies), promote fusion (where bones are induced to grow together into a single, solid mass), or stabilize the sites of fractures. More recently, processed bone has been developed into shapes for use in new surgical applications, or as new materials for implants that were historically made of non-biologically derived materials.
Osteoimplants come in a variety of shapes and sizes including cut cross-sections, cylindrical dowels, cortical rings, elongated struts, wedges, blocks, screws, pins, etc., as well as assembled implants made of two or more bone pieces such as, for example, described in U.S. Pat. No. 5,899,939 to Boyce et al., U.S. Pat. No. 6,025,538 to Yaccarino, III, U.S. Pat. No. 6,123,731 to Boyce et al., and U.S. Pat. No. 6,200,347 B1 to Anderson et al., the contents of each being incorporated herein by reference. Osteoimplants are used in a variety of different surgical procedures including bone fracture repair, spinal fusion procedures, tendon repair, cosmetic surgery, etc. Typically, osteoimplants will include engagement structure formed integrally therein for detachable engagement of an implant insertion tool to facilitate insertion of the osteoimplant into an implant site. Such engagement structure may include a threaded bore, multiple bore holes, a hexagonal recess, an irregular shape recess, etc. For accurate insertion of the osteoimplant at the surgical site, it is important that close tolerances be maintained between the implant insertion tool and the engagement structure of the osteoimplant.
Bone grafting applications are differentiated by the requirements of the skeletal site. Certain applications require a “structural graft” in which one role of the graft is to provide mechanical or structural support to the site. Such grafts contain a substantial portion of mineralized bone tissue to provide the strength needed for load-bearing. Examples of applications requiring a “structural graft” include intercalary grafts, spinal fusion, joint plateaus, joint fusions, large bone reconstructions, etc. Other applications require an “osteogenic graft” in which one role of the graft is to enhance or accelerate the growth of new bone tissue at the site. Such grafts contain a substantial portion of demineralized bone tissue to improve the osteoinductivity needed for growth of new bone tissue. Examples of applications requiring “osteogenic graft” include deficit filling, spinal fusions, joint fusions, etc. Grafts may also have other beneficial biological properties, such as, for example, serving as delivery vehicles for bioactive substances. Bioactive substances include physiologically or pharmacologically active substances that act locally or systemically in the host.
When mineralized bone is used in osteoimplants, it is primarily because of its inherent strength, i.e., its load-bearing ability at the recipient site. The biomechanical properties of osteoimplants upon implantation are determined by many factors, including the specific site from which the bone used to make the osteoimplant is taken; the age, sex, and physical characteristics of the donor; and the method chosen to prepare, preserve, and store the bone prior to implantation, as well as the type of loading to which the graft is subjected.
Structural osteoimplants are conventionally made by processing, and then machining or otherwise shaping cortical bones collected for transplant purposes. Cortical bone can be configured into a wide variety of configurations depending on the particular application for the structural osteoimplant. Structural osteoimplants are often provided with intricate geometries, e.g., series of steps; concave or convex surfaces; tapered surfaces; flat surfaces; surfaces for engaging corresponding surfaces of adjacent bone, tools, or implants, hex shaped recesses, threaded holes; serrations, etc.
One problem associated with many structural osteoimplants is that they are never fully incorporated by remodeling and replacement with host tissue. This is due, in part, to the difficulty with which the host's blood supply may penetrate cortical bone. Moreover, non-demineralized bone is not osteoinductive. Since repair is a cellular-mediated process, dead (non-cellular, allograft or xenograft) bone is unable to repair itself. When the graft is penetrated by host cells and host tissue is formed, the graft is then capable of repair. It has been observed that fatigue damage is usually the result of a buildup of unrepaired damage in the tissue. Therefore, to the extent that the implant is incorporated and replaced by living host bone tissue, the body can then recognize and repair damage, thus eliminating failure by fatigue. In applications where the mechanical load-bearing requirements of the osteoimplant are challenging, e.g., intervertebral spinal implants for spinal fusion, lack of substantially complete replacement by host bone tissue may compromise the osteoimplant by subjecting it to repeated loading and cumulative unrepaired damage in the tissue (mechanical fatigue) within the implant material. Thus, it is highly desirable that the osteoimplant has the capacity to support load initially and be capable of gradually transferring this load to the host bone tissue as it remodels the implant.
As stated above, a known technique for promoting the process of incorporation of osteoimplants is demineralization. The process of demineralizing bone grafts is well known. In this regard see, Lewandrowski et al., J. Biomed Materials Res, 31, pp. 365–372 (1996); Lewandrowski et al., Calcified Tiss. Int., 61, pp. 294–297 (1997); Lewandrowski et al., J. Ortho. Res., 15, pp. 748–756 (1997), the contents of each of which is incorporated herein by reference. However, the prior art has not addressed the need to provide a demineralized osteoimplant with sufficiently mineralized regions for engagement of insertion instrumentation.
Demineralizing bone, using for example, a controlled acid treatment, increases the osteoinductive characteristics of the osteoimplant. Demineralization also provides the osteoimplant with a degree of flexibility. However, removal of the mineral components of bone significantly reduces mechanical strength of bone tissue. See, Lewandrowski et al., Clinical Ortho. Rel. Res., 317, pp. 254–262 (1995). Thus, demineralization sacrifices some of the load-bearing capacity of mineralized cortical bone and as such is not suitable for all osteoimplant designs. Another disadvantage to the demineralization process is the likelihood of creating dimensional changes in the osteoimplant. Demineralization of the bone will ordinarily result in bone of slightly smaller dimensions. Such changes of dimension can make it difficult for a configured piece to mechanically engage with surgical instruments, other implants, or the prepared surgical site.
Accordingly, a need exists for an improved process for demineralizing an osteoimplant to achieve an improved biologic response to the implant while maintaining a mineralized portion of the osteoimplant that can endure mechanical forces and/or maintain close tolerances with insertion instrumentation and/or the implant site. Complete mineralization may be especially important for portions of an osteoimplant experiencing the greatest mechanical loads such as engagement structure of the osteoimplant. In certain embodiments, these areas are masked from the demineralization process. In other embodiments, a demineralized portion of the osteoimplant is removed by, for example, configuring, to expose the mineralized portion beneath the demineralized surface region.
It would be advantageous if a surface demineralized load-bearing osteoinductive osteoimplant could be achieved efficiently and accurately by a simple process. Use of such an osteoimplant in a load-bearing procedure such as, for example, joint plateau revisions, joint fusions, spinal fusions, long bone reconstructions, etc. would provide a favorable outcome for the recipient of the implant.